1. Field of the Invention
This invention relates to optical communications systems, in particular, to wavelength conversion. Still more particularly, the present invention relates to wavelength conversion using vertical lasing semiconductor optical amplifiers (VLSOA) and other lasing semiconductor optical amplifiers.
2. Background of the Invention
As the result of continuous advances in technology, particularly in the area of networking such as the Internet, there is an increasing demand for communications bandwidth. For example, the transmission of data over a telephone company's trunk lines, the transmission of images or video over the Internet, the transfer of large amounts of data as might be required in transaction processing, or videoconferencing implemented over a public telephone network typically require the high speed transmission of large amounts of data. As applications such as these become more prevalent, the demand for communications bandwidth capacity will only increase.
Optical fiber is a transmission medium that is well suited to meet this increasing demand. Optical fiber has an inherent bandwidth that is much greater than metal-based conductors, such as twisted pair or coaxial cable; and protocols such as the SONET optical carrier (OC) protocols have been developed for the transmission of data over optical fibers. Typical communications system based on optical fibers include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and then transmits the resulting optical signal via the optical fiber to the receiver. The receiver recovers the original data from the received optical signal.
To maximize the amount of data transmitted on a single fiber, many different wavelengths of light are carried over the same optical fiber. Each wavelength acts as a different channel, carrying its own signal. This allows the data capacity of a single optical fiber to be greatly increased. When data in a network must be switched, often the signals from two separate fiber inputs must be output in a single fiber. If the signals are carried by the fiber inputs on the same wavelength, a problem known as signal blocking results. In such a case, if both signals remain on the same wavelength and are output in the single output fiber, interference and loss of data will result.
FIG. 1 is a block diagram of a prior art switch where such a problem may occur. A signal on a first wavelength (“λ1”) enters the switch 8 on a first fiber 2. A second signal carried on the same first wavelength λ1 enters the switch 8 on a second fiber 4. The signals must both be output on a single output fiber 10. In order for the output fiber 10 to carry both signals without blocking, one of the signals on the first wavelength λ1 must be converted to a second wavelength (“λ2”). Otherwise, interference and loss of data will result. In the past, wavelength conversion has been accomplished through optical-electrical-optical (“OEO”) systems and Mach Zhender systems.
FIG. 2a is a block diagram of a prior art OEO system for wavelength conversion. In an OEO wavelength conversion system, the input is optical, the wavelength conversion happens electrically, and then the signal is output optically. As seen in FIG. 2a, the signal is carried in an input optical fiber 14 on a first wavelength λ1. An optical detector 16 detects the signal and outputs the signal in electrical form to an electrical remodulator 18. The electrical remodulator 18 remodulates the signal so that the signal is carried by a new wavelength λ2. A source 20 converts the signal from the electrical domain to the optical domain and resends the signal, now on second wavelength λ2 on an optical fiber output 22. The signal remains the same in both the input 14 and the output 22, but the wavelength that carries the signal changes. Because the signal must be converted from optical to electrical, then converted back to optical, the system has the drawbacks of being relatively large, complex and expensive, and further suffers losses from the conversion between electrical and optical.
FIG. 2b is a block diagram of a Mach Zhender modulator that converts a signal from one wavelength to another. The signal is carried by a first wavelength λ1 on an input optical fiber 24. A second optical fiber input 26 carries a second signal at a second wavelength λ2. A splitter 28 or alternatively a directional coupler splits the second signal into two parts. The signal on the first wavelength λ1 is combined with one part of the second signal at a combiner 38 and this combination enters an amplifier 30 such as a semiconductor optical amplifier (“SOA”). Within the amplifier 30, crosstalk occurs between the signal on the first wavelength λ1 and the second wavelength λ2. The other part of the second signal enters a second amplifier 32. The outputs of the first amplifier 30 and second amplifier 32 are combined at a combiner 34 and output on an optical fiber 36. Due to the interference between the first and second wavelengths, the signal is carried by the second wavelength λ2 in the resulting combined output. However, because the Mach Zhender uses multiple amplifiers and requires an independent second wavelength input, the system has the drawbacks of being relatively large, complex and expensive. The Mach Zhender is also subject to cross-phase modulation and gain saturation.
Systems such as those described above often require pre-amplifiers to boost the signal input to the wavelength converter to counter signal loss when converted. These pre-amplifiers add more size and expense to the wavelength converter.
Many applications require an array of wavelength converters for many different wavelengths. In such applications, extra wavelength converters must be available to replace those that have failed. Storing a different wavelength converter for each wavelength is expensive.